1. Field of the Invention
The present invention relates to an f.theta. lens using a rotary polygon mirror and a lens for forming a linear image and using a rotary polygon mirror.
2. Description of the Related Art
In a known optical scanner, a light beam is emitted from a semiconductor laser and is changed to a parallel light beam by a collimator lens. This parallel light beam is formed by a first image forming optical system as a linear image extending in a main scan-corresponding direction. This linear image is deflected at an equal angular velocity by a rotary polygon mirror having a deflection reflecting face in the vicinity of the linear image. The deflected light beam is formed as a light spot on a scanned face by a second image forming optical system so as to perform an optical scanning operation. An f.theta. lens is used as the second image forming optical system in such an optical scanner. The f.theta. lens has a function for providing an equal moving speed of the light spot on the scanned face. The f.theta. lens also has a function for correcting an inclination of the reflecting face of the rotary polygon mirror.
In the above optical scanner, the semiconductor laser as a light source is heated by a driving electric current flowing therethrough so that the temperature of the semiconductor laser is changed. In this case, a luminous wavelength of the semiconductor laser is generally changed by .+-.2 nm by a so-called mode hop. Accordingly, when no chromatic aberration of the f.theta. lens with respect to magnification is corrected, a writing position of the optical scanner with respect to the light spot is shifted by the change in wavelength of the light beam emitted from the semiconductor laser, thereby causing a so-called jitter. Such a jitter has a serious bad influence upon the quality of a recording image provided by the optical scanning operation of high density.
Various kinds of f.theta. lenses for correcting the chromatic aberration with respect to magnification are known as shown in e.g., Japanese Patent Application Laying Open (KOKAI) No. 59-7918. However, such f.theta. lenses cannot correct the above inclination of a reflecting face of the rotary polygon mirror.
With respect to the entire optical scanner, to effectively correct the chromatic aberration of the f.theta. lens, it is preferable to further correct the chromatic aberration on an optical axis of the first image forming optical system for forming the parallel light beam from the light source as a linear image. When no chromatic aberration on the optical axis of the first image forming optical system is corrected, the position of the formed linear image is shifted from a designed position thereof when the luminous wavelength of the semiconductor laser is different from a designed wavelength. Accordingly, field curvature in a cross scanning direction is changed in accordance with the difference in wavelength even when the chromatic aberration of the f.theta. lens with respect to magnification is corrected. Therefore, the diameter of a light spot is changed in the cross scanning direction in accordance with an image height so that it is difficult to perform the optical scanning operation of high density.